In our laboratory we have been developing a new approach to the investigation of the basic biochemical defect in cystic fibrosis (CF) using cell culture model. In our initial studies, fibroblasts from patients with CF showed more resistance to the cytotoxic effects of ouabain, an inhibitor of ion transport, then cells from normal individuals when survival was measured in K ion free medium. However, ion transport in CF and normal cells was inhibited to the same degree by ouabain which suggested ouabain resistance in CF cells might not be due to an alteration in their membrane Na ion, K ion ATPase. In subsequent studies, we explored other potential drug resistance markers which might be related to ouabain resistance. Survival experiments with ethacrynic acid, another ion transport inhibitor, and with colchicine and aminopterin, drugs which interact at the cell membrane, were performed. The CF and normal cells had identical sensitivity to all these agents in medium with and without K ion. However, survival studies with dexamethasone, a glucocorticoid with a sterol nucleus structure similar to that of ouabain, showed that CF cell had increased resistance to this drug independent of K ion concentration in the medium. In this proposal, we outline investigations into the basic molecular mechanisms by which glucocorticoids exert their biological effects in normal and CF fibroblasts. If a specific molecular defect in glucocorticoid metabolism is found in CF cells, it could help explain the phenotype of dexamethasone resistance and may provide an insight into the basic genetic defect in CF.